The metallurgy of silicon devices is well established as evidenced by the large number of patents and the volume of papers concerning the detailed behavior of the metal-semiconductor interface. In contrast, methods of metallizing GaAs and other compound semiconductors are not well developed and the behavior of the interface is under current examination. Similarly, solid phase epitaxy (SPE) of silicon on silicon or silicides on silicon have been extensively studied, whereas SPE on compound semiconductors is still under investigation. Epitaxial growth by liquid and vapor phase techniques (high temperature) on compound semiconductors is a well established field relative to that which is known for SPE in these materials.
Problems with presently available ohmic contacts include island formation or balling-up at the surface and deep penetration or spiking of the metal into the GaAs substrate. Ohmic contacts to n-GaAs in current use involve a so-called alloying procedure which consists of melting a eutectic Au-Ge or Sn-based alloy film on the GaAs. The alloying cycle is critical in attempts to achieve low specific contact resistance. The cycle is sufficiently rapid that the equilibrium reactions are not reached and hence the contact is metastable, resulting in degradation upon further heating and use. Contacts of this type are susceptible to problems of reliability for the devices which have incorporated them. Methods which raise the temperature of the substrate can cause particle precipitation, increased spiking and increased contact degradation. For example, contacts to GaAs must withstand annealing temperatures up to 850.degree. C. for post ion implantation heat treatment. However, GaAs exhibits signs of degradation at temperatures above 600.degree. C. and the contact itself would degrade before the GaAs.
Ohmic contacts to n-GaAs using a Ge/GaAs heterojunction have been developed by using molecular beam epitaxy (MBE) with specific contact resistances below 10.sup.-7 .OMEGA.cm.sup.2, R. A. Stall et al, "A Study of Ge/GaAs Interfaces Grown by Molecular Beam Epitaxy", J. Appl. Phys. 52(6), June 1981, pp. 4062-4069. Au metallization can then be evaporated onto the heavily arsenic-doped Ge layers. The problem with MBE is that the substrate must be heated to high temperatures prior to growth of the Ge in order to properly clean and prepare the GaAs surface and is held at elevated temperatures during the Ge growth. This makes the technique unsuitable for producing electrical contacts on GaAs which will require lithographic processing. A wafer of GaAs with property modifying and device creating entities such as diffusions, implants, pattern, other metal layers, and photoresists cannot be subjected to MBE without changing or destroying the previous steps.
Epitaxial growth of Ge on GaAs has been obtained by raising the temperature to high enough values to result in liquid phase growth. Ge which has been rendered amorphous by implantation can be raised to a temperature of approximately 375.degree. C., at which point the amorphous Ge will regrow epitaxially in the solid phase. SPE enables greater control of the geometry and the location of the epitaxial layers which is often difficult with liquid or vapor phase technologies, S. S. Lau and W. F. Van der Weg, "Thin Films--Interdiffusion and Reactions", edited by J. M. Poate, K. N. Tu, J. W. Mayer (Wiley: N.Y., 1978) Chapter 12. Until the present invention, it has not been possible to cause a deposited layer of Ge on an unheated GaAs surface to grow epitaxially at low temperatures.
Specific contact resistance of 3.times.10.sup.-5 .OMEGA.cm.sup.2 has been reported with Ni layered over Ge deposition on a GaAs substrate. W. T. Anderson et al, "Smooth and Continuous Ohmic Contacts to GaAs Using Epitaxial Ge Films", J. Appl. Phys. 49(5), May 1978, pp. 2998-3000. The GaAs substrate is heated to 575.degree. C. for 15 min under a pressure of 2.times.10.sup.-7 torr to desorb the surface oxides just prior to deposition of the Ge layer. A reduction of surface oxygen content of 2.5 times was shown. The Ge deposition of 100 nm on a heated GaAs substrate was followed by a 100 nm Ni overlay. Then the combination was sintered at 450.degree. to 550.degree. C. for times from 1 to 45 min.
Specific contact resistance of 2.times.10.sup.-4 .OMEGA.cm.sup.2 was reported by Lau at the University of California at San Diego. E. D. Marshall et al, "Pt and Pd Silicides and Pd Germanide as Contact Metallizations for GaAs", Mat. Res. Soc. Symp. Proc. Vol 25 (1984), pp. 63-68. To avoid the interfacial oxide problem a Pd layer is deposited between the Ge and the GaAs. The Pd layer is believed to react with the interfacial oxide and some of the Ge to form PdGe. The excess Ge is free to diffuse through the PdGe and grow by solid phase epitaxy on the GaAs. The resulting epitaxy using this technique is of poor quality.
Nothing in the prior art teaches solid phase epitaxial growth of deposited Ge layers on a GaAs substrate directly at temperatures low enough to preserve the integrity of prior steps, which is disclosed in the present invention.